Inteferometric synthetic sonar antenna

ABSTRACT

The invention relates to an interferometric synthetic sonar antenna. It consists in processing the signals coming from the sensors of two superimposed parallel arrays forming the reception antenna, these two arrays being offset by a half-pitch between sensors, relative to each other and parallel to themselves. In a first mode of operation, the signals of the two arrays are processed to form a single synthetic antenna having a higher resolution in bearing than that of the two synthetic antennas of the interferomatric mode. In a second mode of operation, the signals of the two arrays are processes jointly in the auxiliary transmission frequency bands, enabling the two synthetic antennas to be self-calibrated with improved precision.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on International Application No.PCT/FR01/03827, filed on Dec. 4, 2001 entitled “InterferometricSynthetic Sonar Antenna”, which in turn corresponds to FrenchApplication FR 00/16591 filed on Dec. 19, 2000, and priority is herebyclaimed under 35 USC 119 based on these applications. Each of theseapplications are hereby incorporated by reference in their entirety intothis application.

The present invention relates to interferometric sonar antennas used toform a synthetic antenna and, more particularly, to the self-calibrationof a synthetic antenna of this kind.

Synthetic antennas are well-known in the field of both radar and sonardevices, and the self-calibration of such antennas is itself a prior arttechnique described especially in an article by Didier BILLON and FranckFOHANNO (of THOMSON MARCONI SONAR SAS) in the “Proceedings of OCEAN 98”,IEEE, pages 965–970.

SUMMARY OF THE INVENTION

The principle of the invention lies in:

-   -   offsetting the two arrays of the interferometer relative to each        other and parallel to themselves by a distance equal to half the        pitch between sensors so as to enable the broadening of the        transmission sector without increasing the number of sensors of        the antenna;    -   forming an array known as a unified array by processing the        signals of the sensors of the two arrays.

This principle enhances the resolution in bearing of the syntheticantenna processing operations.

The invention proposes an interferometric synthetic sonar antenna, thereception antenna of which comprises two superimposed, identicalparallel linear arrays with a constant pitch between sensors,characterized in that the two arrays (101,102) are offset relative toeach other and parallel to themselves by a distance equal to half thepitch between the sensors, and in that the signals coming from thesesensors are processed to form an array known as a unified array inestimating the propagation delay time between the two arrays, theunified array being used in synthetic antenna processing.

BRIEF DESCRIPTION OF THE DRAWING

Other particular features and advantages of the invention shall appearmore clearly from the following description, given by way of anon-restrictive example and made with reference to the appended figurewhich is a drawing of the reception antenna with offset arrays.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter in the description, the term “antenna” used by itselfpertains to the “reception antenna”.

The self-calibration of a synthetic antenna, based on inter-recurrencecorrelation, consists in estimating a length L of a shift parallel tothe antenna, a delay τ between two correlated signals, and a variationin bearing β induced by the rotation of the antenna, the estimationbeing done from signals of two successive recurrences received on twotime intervals corresponding to a same distance interval and smallenough for the assumption to be made that one of these parameters isconstant. The length L is the sum, firstly, of the lengths of thecomponents, parallel to the antenna, of the shift between the twosuccessive instants of transmission and, secondly, of the shift betweentwo instants of reception relative to the center of the distanceinterval considered. If Oξ is the axis oriented in the direction of theshift, the self-calibration is based on the correlation between thesignal received at the abscissa point ξ at the first recurrence and atthe abscissa point ξ−L at the second recurrence.

The precision of the estimates depends especially on the number of pairsof abscissa values on the antenna (ξ,ξ−L) such that the incidentacoustic field generated by the reverberation of the seabed hasindependent values at the abscissa points ξ included between ξ₁+L andξ_(M), where ξ₁ and ξ_(M) are the abscissa values of the phase centersof the far-end sensors of the antenna.

The number of independent values of the acoustic field along thereception antenna is equal to the ratio between the length of theantenna and the length of correlation of the field along the antenna,which is equal to the ratio λ/Δθ between the wavelength λ and the widthin bearing Δθ of the transmission sector. This width of the transmissionsector furthermore determines the resolution limit of the syntheticantenna:$\delta_{\min} = \frac{\lambda}{2\left( {{\Delta\;\theta} - {\Delta\;\varphi}} \right)}$where Δφ is the equivalent reduction of the transmission sector causedby the rotation of the platform during the period of formation of thesynthetic antenna.

The pitch between the sensors of the reception antenna should be smallerthan this correlation length of the field, the ratio between the twolengths having, in practice, a value of about 1.5. In other words, thenumber of sensors must be greater, by the same ratio, than the number ofindependent values of the incident field along the antenna.

Thus, in the prior art, increasing the number of independent values ofthe field along the antenna implies reducing the pitch between sensors,and hence increasing the number of sensors. The invention seeks toobtain the same effect without increasing the number of sensors.

According to the prior art, for a sonar system using an interferometricsynthetic antenna, two superimposed, parallel, identical linear arraysare used, one obtained by translation of the other along an axis Oζperpendicular to the two arrays.

According to the invention, as shown in the figure, this configurationis modified by offsetting the two arrays 101, 102 parallel tothemselves, along the axis Oξ, by half the pitch between sensors. Thus,from the signals of the two arrays, the device of the inventiongenerates signals of a linear array having a same length but havingtwice as many sensors as each of the two arrays, hence having a pitchthat is halved in making use of the known relationship:s _(sup)(ξ,t)≈s _(inf)(ξ,t−τ _(int)(t))·exp(−j 2πf ₀τ_(int)(t))where s_(inf)(ξ,t) and s_(sup)(ξ,t) are the complex amplitudes of thesignals of sensors whose phase centers would have the abscissa value ξrespectively on the lower array and the upper array, f₀ is the frequencyof demodulation equal to the central frequency of transmission, andτ_(int)(t) is the propagation delay time between the two arrays.

To simplify the explanation, it is assumed that the variation of theangle between a direction of arrival and the axis Oζ (angle ofelevation) in the sector of the incident field at a fixed instant t issmall enough to overlook the corresponding variation of the delayτ_(int)(t), which then should be far below 1/f₀. This hypothesis, whichdoes not restrict the general scope of invention in general, is mostfrequently met in practice.

According to the invention, the signals of the array constituted by theM sensors of the upper array, having their phase centers at the abscissapoints ξ_(sup,1), . . . ,ξ_(sup,M) and M intermediate sensors whosephase centers would have the same abscissa values ξ_(inf,1), . . .,ξ_(inf,M) as those of the phase centers of the lower array, areprocessed in the manner now described.

It is assumed, for example, that the shift is in the reverse directionof Oξ as in the figure. Let s_(m)(t), 1≦m≦M, be the complex amplitudesdefined by the expressionss _(m)(t)=s _(sup)(ξ_(sup,1+(m−1)/2) ,t) for m as an odd-parity values _(m)(t)=s_(inf)(ξ_(inf,m/2) ,t−{circumflex over (τ)}_(int)(t)).exp(−j2πf ₀{circumflex over (τ)}_(int)(t)) for m as aneven-parity value.

The processing of the signals coming from the sensors of the two arraysconsists in computing an estimate {circumflex over (τ)}_(int)(t) of thedelay, τ_(int)(t) for example, by means of the expression:$\begin{matrix}{{{\hat{\tau}}_{int}(t)} = {\arg\;{\max\limits_{\tau}{{Re}\left( {\int_{t - {\Delta\;{t/2}}}^{t + {\Delta\;{t/2}}}{\int_{{- \Delta}\;{\theta/2}}^{{+ \Delta}\;{\theta/2}}{{S_{\sup}\left( {\theta,u} \right)} \cdot {S_{\inf}\left( {\theta,{u - \tau}} \right)}^{*} \cdot}}} \right.}}}} \\\left. {{\exp\left( {j\; 2\;\pi\; f_{0}\tau} \right)} \cdot {\mathbb{d}\theta} \cdot {\mathbb{d}u}} \right)\end{matrix}$where S_(inf)(θ,t) and S_(sup)(θ,t) are the signals of a channel of thelower antenna and a channel of the upper antenna oriented in bearing θ,their phase centers have a same abscissa value on the axis Oξ.

In general, for any array parallel to the two physical arrays andlocated in the same plane, we have the relationships:s _(m)(t)=s_(sup)(ξ_(sup,1+(m−1)/2) ,t+{circumflex over (τ)}_(int,sup)(t)).exp(j2πf ₀{circumflex over (τ)}_(int,sup)(t)) for m as anodd-parity value, ands _(m)(t)=s_(inf)(ξ_(inf,m/2) ,t−{circumflex over (τ)}_(int,inf)(t)).exp(−j2πf ₀{circumflex over (τ)}_(int,inf)(t)) for m asan even-party value.with{circumflex over (τ)}_(int,inf)(t)+{circumflex over(τ)}_(int,sup)(t)={circumflex over (τ)}_(int)(t).

For convenience of terminology, the inventor has called this kind ofarray a “unified array”. The invention proposes to exploit this arrayaccording to two distinct modes of operation.

In a first mode of operation, the unified array improves the resolutionof the synthetic antenna in imagery alone. Indeed, an interferometricsynthetic antenna sonar can fulfil two functions simultaneously: imagingfunctions which consist in producing images of the intensity of theacoustic backscattering from the seabed, and bathymetry functions inwhich relief maps of the seabed are produced. These two functionsbenefit from the resolution in bearing of the synthetic antennaprocessing.

The invention therefore proposes an additional mode where, as atrade-off for giving up interferometry, imagery with improved resolutionis obtained as compared with the prior art mode where the two functionscoexist. In this additional mode, the transmission sector is broadened.This broadening is obtained in a known way by diminishing the width ofthe pupil of the transmission antenna, which may be distinct from thereception antenna. The width of the transmission sector, hence thenumber of independent values of the field along the antenna, may forexample be doubled. The synthetic antenna processing is then performedon the unified array in the same way as would be done on an array reallyhaving twice as many sensors as each of the two interferometry arrays.

The second mode of operation relates to synthetic antenna techniquesusing auxiliary transmission as described in the U.S. Pat. No. 9,510,953filed on 19 Sep. 1995 by THOMSON-CSF.

We shall therefore consider an interferometer with offset arraysassociated with an auxiliary transmission device. The self-calibrationparameters. L and β of each of the two synthetic antennas associatedwith the two arrays are identical. As for the third parameters,respectively referenced τ_(inf) and τ_(sup) for the lower syntheticantenna and the upper synthetic antenna, they comply with therelationship:τ_(inf)=τ_(sup)−τ_(int,2)+τ_(int,1)

where τ_(int,1) and τ_(int,2) are the propagation delay times betweenthe two arrays at the first and second recurrences of the pair of twosuccessive recurrences processed by the self-calibration, as describedin the article by the present inventor in Proceedings of the EuropeanConference on Underwaters Acoustics 2000, pp. 419–424.

The bearing sector of the auxiliary transmissions, which are the onlytransmissions exploited by the self-calibration, is broader than in theprior art. This amounts to having a transmission antenna length that issmaller than in the prior art. As for the width of the bearing sector ofthe main transmission used for the formation of the synthetic antenna,it remains the same.

The self-calibration is obtained solely on the unified array, whosesignals are generated as described here above. If, for example, thewidth of the bearing sector of the auxiliary transmissions is doubled,the precision of spatial interpolation on the unified array is the sameas the one obtained previously on the two arrays. In the example wherethe unified array has the same coordinate on Oζ as the upper array, theestimates of L, τ_(sup) and β thus obtained are more precise than thosethat would be obtained on this upper array with the classic method.τ_(int,1) and τ_(int,2) also estimated from the signals of the twoarrays, separately on the two recurrences. This estimation will beappreciably more precise than that of τ_(sup) because, firstly, thecorrelation of the acoustic fields of the two antennas on a samerecurrence is generally higher than the inter-recurrence correlation ofthe acoustic fields of a same antenna and, secondly, it may make use ofthe signals at the main frequency and at the two auxiliary frequencies.Consequently, by means of the previous relationship, an estimate ofτ_(inf) is obtained, the error of this estimate having, as its maincomponent, the error in the estimate of τ_(sup).

The gain in resolution enabled by this method essentially results fromthe greater precision of estimation of the parameter β (reference couldbe made to the article cited on page 1). The gain is particularly highwhen the intersection of the sectors of the auxiliary transmissions ofthe two successive recurrences, which represent the angular sector thatcan be exploited by self-calibration, is greatly reduced by the rotationof the platform. In this case, doubling the width of the transmissionsector more than doubles the number of independent values of theacoustic field along the antenna. This consideration can be applied alsoto the first mode of operation of the unified array described furtherabove.

Let us assume, for example, that the desired resolution δ_(min) is equalto 30λ, that the duration of recurrence is T_(Γ)=2 s (range 1500 m), andthat the platform is given a sinusoidal rotation with an amplitudeφ_(max)=2° and a period T_(φ)=6 s in the aiming plane. The maximumrotation during the formation of the synthetic antenna Δφ is equal to2φ_(max)=4°. The width of the transmission sector necessary for theformation of the synthetic antenna is Δθ=λ/2δ_(min)+Δφ or 5°. Theminimum width of the sector exploited by self-calibration, which is theintersection of the sectors of two successive transmissions, is:${\Delta\;\theta} - \underset{\underset{{\approx 3},5^{\circ}}{︸}}{2\varphi_{\max}{\sin\left( {\pi\frac{T_{r}}{T_{\varphi}}} \right)}}$giving, in this example, 1.5°.

With the method according to the invention, the width of the sector ofthe auxiliary transmissions can then be 10° in this example, while thatof the sector exploitable by self-calibration is at least 10°−3.5°=6.5°,i.e. more than four times the minimum width of the sector exploited inthe prior art method. The mean standard deviation of the error affectinga self-calibration parameter is inversely proportional to the squareroot of the width of the sector exploited by the self-calibration ifthis width is sufficient. Thus, it is reduced by a factor of up to twoin the example chosen. In the unfavourable case where the rotation ofthe antenna excessively reduces the width of the sector exploited byself-calibration, the means standard deviation varies more rapidly as afunction of this width, and hence the gain in precision provided by theinvention is even greater.

1. An interferometric synthetic sonar antenna of the type comprising: areception antenna including two superimposed, identical parallel lineararrays with a constant pitch between sensors; said two arrays are offsetrelative to each other and parallel to themselves by a distance equal tohalf the pitch between the sensors; wherein the signals coming fromthese sensors are processed to form an array known as a unified array inestimating the propagation delay time between the two arrays, theunified array being used in a synthetic antenna processing operation;wherein, in a particular mode of non-interferometric imaging, thetransmission sector in bearing, is broadened and the unified array isexploited for the formation of a synthetic antenna.
 2. The antennaaccording to claim 1, using auxiliary transmissions to carry out theself-calibration: wherein the bearing sector of at least one auxiliarytransmission is broader than the one corresponding to the maintransmission, and wherein the unified array is exploited to carry outthe self-calibration of the two synthetic antennas formed with thereceived signals resulting from the main transmission.
 3. The antennaaccording to claim 1, wherein, in another particular mode, auxiliarytransmissions are used to carry out the self-calibration.
 4. Aninterferometric synthetic sonar antenna comprising: a reception antennaincluding two superimposed, identical parallel linear arrays with aconstant pitch between sensors; said two arrays are offset relative toeach other and parallel to themselves by a distance equal to half thepitch between the sensors; the signals coming from these sensors areprocessed to form an array known as a unified array in estimating thepropagation delay time between the two arrays, the unified array beingused in a synthetic antenna processing operation; wherein in a firstparticular mode of non-interferometric imaging, the transmission sector,in bearing, is broadened and in that the unified array is exploited forthe formation of a synthetic antenna; wherein in another particularmode, auxiliary transmissions are used to carry out theself-calibration; the bearing sector of at least one auxiliarytransmission being broader than the one corresponding to the maintransmission, and the unified array being exploited to carry out theself-calibration of the two synthetic antennas formed with the receivedsignals resulting from the main transmission.
 5. An interferometricsynthetic sonar antenna of the type comprising: a reception antennaincluding two superimposed, identical parallel linear arrays with aconstant pitch between sensors; said two arrays are offset relative toeach other and parallel to themselves by a distance equal to half thepitch between the sensors; wherein the signals coming from these sensorsare processed to form an array known as a unified array in estimatingthe propagation delay time between the two arrays, the unified arraybeing used in a synthetic antenna processing operation; wherein, in aparticular mode of non-interferometric imaging, the transmission sectorin bearing, is broadened and the unified array is exploited for theformation of a synthetic antenna, wherein the bearing sector of at leastone auxiliary transmission is broader than the one corresponding to themain transmission, and wherein the unified array is exploited to carryout the self-calibration of the two synthetic antennas formed with thereceived signals resulting from the main transmission.
 6. The antenna ofclaim 5, wherein, in a particular mode of non-interferometric imaging,the transmission sector in bearing is broadened and the unified array isexploited for the formation of a synthetic antenna.